1. Field of the Invention
The present invention relates to high-output permanent magnet motors incorporating a doubly salient structure with stationary permanent magnets in the stator and, more particularly, to field weakening via controlled movement of steel insets toward/against the sides of the stator near the permanent magnets to provide a controllable by-pass flux path thereabout. Appropriate control broadens the constant power range of the motor.
2. Description of the Background
In a variable reluctance motor (VRM), torque is generated by a doubly-salient rotor and stator which tend to align themselves in order to reach a position of minimum magnetic reluctance. Under ideal switching conditions, VRMs have the potential to be highly efficient. Unfortunately, actual switching conditions are less than ideal, and attempts to achieve such conditions have resulted in commercially impractical converter circuits with excessive voltage and current stresses imparted to the switching devices. In a drive utilizing a variable reluctance motor, the current in each phase should be decreased to zero immediately when the rotor is aligned with that phase. The problem lies in the existence of a large turn-off inductance. The phase inductance is a maximum when the rotor is aligned with that phase. Since this large inductance will prevent rapid decrease of the current due to the energy stored in the magnetic field, the current in each phase cannot immediately be decreased to zero when the rotor reaches alignment. Consequently, the decaying residual current induces a detrimental reverse-torque as the rotor pole surpasses alignment with the corresponding stator pole. The problem is most serious when the speed of the motor is high.
One solution to the above-described problem was provided in U.S. patent application Ser. No. 07/926,765, filed Aug. 6, 1992. A new type of permanent magnet motor was therein disclosed which incorporated a doubly salient structure with stationary permanent magnets in the stator.
FIG. 1 shows a cross-section of a doubly salient stationary permanent magnet motor (DS.sup.2 PM) as set forth in U.S. patent application Ser. No. 07/926,765, filed Aug. 6, 1992.
A stator 10 consists of a plurality of discrete laminated layers, each layer being punched to form six salient (or projecting) poles 12 positioned at angular intervals .theta..sub.s of .pi./3 radians. Each pole 12 has a pole arc .theta..sub.ps of .pi./6 radians.
The rotor 16 also consists of a plurality of discrete laminated layers each of which is punched to form four salient poles 18 positioned at angular intervals .theta..sub.r of .pi./2. Each pole 18 also has a pole arc .theta..sub.pr equal to or slightly greater than .pi./6 radians.
In the DS.sup.2 PM embodiment of FIG. 1, stator 10 is wound with three short pitch windings corresponding to three phases A-C. Each short pitch winding (for example, the winding of phase A) further comprises two short pitch coils A.sub.1-2 connected in series, and the coils A.sub.1 and A.sub.2 of each winding are wound around a diametrically opposite pair of stator poles 12.
In addition, two permanent magnets 22 and 24 are embedded inside the stator 10. The inclusion of permanent magnets 22 and 24 in the stator 10 rather than the rotor 16 has distinct advantages in that the motor is able to run at higher speed, the motor may be manufactured at a lower manufacturing cost, and the motor lends itself to better field weakening operation performance.
The permanent magnets 22 and 24 themselves generate the primary flux, and a secondary (armature reaction) flux is induced by the stator pole windings A-C. Due to their air-like permeability, permanent magnets 22 and 24 present a very high bi-planar reluctance which blocks the ordinary path of the secondary flux through the stator 10.
The particular stator pole 12 and rotor pole 18 arrangement of the present invention ensures that the total overlapped pole area remains constant for all positions of rotor 16. This way, the total air-gap reluctance (which is the primary reluctance for the permanent magnet excitation) is invariant to rotor 16 displacement .theta..sub.d, and there exists a substantially linear transfer of permanent magnet flux between adjacent stator poles 12 during rotation of rotor 16. Consequently, permanent magnets 22 and 24 produce no cogging torque at no load.
Since torque is produced as a result of the change of flux linkage in the active stator winding(s) A-C, there is a reaction torque component caused by the interaction of stator winding current and the permanent magnet flux, and this reaction torque is the dominant driving torque of the motor. There is also a reluctance torque component caused by the variation in the reluctance of the magnetic path of the winding A-C. Hence, the DP.sup.2 VRM works on the variable reluctance principle as well as permanent magnet brushless DC motor principles.
Insofar as the reaction torque component, FIG. 2 shows the variations of the currents i and the corresponding flux linkages .lambda. in each phase resulting from the permanent magnets 22 and 24. Positive current is injected into a given phase when the magnetic flux linking that phase is increasing, and negative current is injected when the magnetic flux is decreasing. Consequently, a positive reaction torque component can be produced over the entire area of overlap of an active stator pole 12 and rotor pole 18 pair.
Insofar as the reluctance torque component, the reluctance torque .tau..sub.er in the respective phases will be of zero average if the amplitudes of the positive and negative pulses of the corresponding phase currents are kept constant and equal to each other by means of pulse width modulation of the associated power converter. In addition, the instant of reversal of the currents i.sub.a, i.sub.b, and i.sub.c applied to the stator windings must be kept centered about the peak of the triangle-shaped flux-linkage variations .lambda..sub.a, .lambda..sub.b, and .lambda..sub.c as shown in FIG. 2.
Under high speed operation, the associated power converter is no longer capable of maintaining the current amplitudes of the positive and negative pulses constant by means of pulse width modulation. In this case, the voltage applied to the motor of FIG. 1 will be simple positive and then negative rectangular pulses over the instants when the desired current should be positive and negative. This is single pulse operation.
Given that the DS.sup.2 VRM will be powered by conventional single pulse operation at high speed, the phase currents will no longer be constant amplitude pulses at high speed. Consequently, the current will peak in the first half stroke where the inductance is increasing and will drop rapidly during the second half stroke where the inductance is decreasing. The uneven distribution of the uneven distribution of the phase current will give rise to net reluctance torque which can be used to extend the constant horsepower speed range. The presence of reluctance torque in addition to reaction torque markedly enhances the torque production during this mode of operation. Consequently, the constant power range can be extended compared to more conventional permanent magnet machines.
The present application proposes an efficient method and apparatus for accomplishing the above-described field weakening at all speeds of the motor.